Compilation error:
  arch/powerpc/perf/perf_regs.c:80:undefined reference to `.is_sier_available'

Currently is_sier_available() is part of core-book3s.c, which is added
to build based on CONFIG_PPC_PERF_CTRS.

A config with CONFIG_PERF_EVENTS and without CONFIG_PPC_PERF_CTRS will
have a build break because of missing is_sier_available().

In practice it only breaks when CONFIG_FSL_EMB_PERF_EVENT=n because
that also guards the usage of is_sier_available(). That only happens
with CONFIG_PPC_BOOK3E_64=y and CONFIG_FSL_SOC_BOOKE=n.

Patch adds is_sier_available() in asm/perf_event.h to fix the build
break for configs missing CONFIG_PPC_PERF_CTRS.

Fixes: 333804dc ("powerpc/perf: Update perf_regs structure to include SIER")
Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
[mpe: Add detail about CONFIG_FSL_SOC_BOOKE]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200614083604.302611-1-maddy@linux.ibm.com